The invention is based on an apparatus for operating an optical rain sensor.
From European Patent Disclosure EP 0 460 180 B1, an apparatus for triggering a wiper system as a function of moisture on a window is already known that has an optoelectronic rain sensor whose sensor signal, on the one hand, has one component based on the presence of moisture on the window (useful light component) and one component based on the ambient light reaching the rain sensor (interfering light component), and on the other includes only a component based on the ambient light (interfering light component). In a first interval, the sensor signal composed of the useful light component and the interfering light component is sampled by first means. Second means sample the sensor signal in a second interval, in which only the interfering light component based on the ambient light is present. The first and second means are sample and hold elements, for example. The signals sampled during the first and second intervals, respectively, are subtracted linearly from one another in a differential amplifier, so that an output signal is obtained that corresponds to the useful light component of the sensor signal. The first and second intervals are assumed to be close together in time. The length of the first interval for detecting the wetting of the window is on the order of magnitude of microseconds, while the second interval for detecting the ambient light is on the order of magnitude of milliseconds.
A disadvantage here is that two signals each have to be processed. This is attained at the cost of resolution of the useful light component. This will now be explained, taking as an example the evaluation of the sensor signal with a microcontroller, but it is also relevant to an analog subtractor: In rain sensors, under certain prerequisites regarding the ambient light, such as the sun going down, it is known that the interfering light component of the sensor signal can far exceed the useful light component. As an example, the interfering light component can be greater than the useful light component by a factor of 10. For an eight-bit microcontroller under such light conditions with maximum amplification of the sensor signal, only about 25 bits are available for the useful light component. In principle, the minimum detectable change in the sensor signal is one bit, and in this case therefore corresponds to a detectable relative change in the useful light signal of four percent. Vehicle manufacturers, however, require that relative signal changes of less than one percent be detected; that is, the signal dynamics of the rain sensor should be better than one percent, so that wiper operation can be attained that is attuned to the wetting of the window and that meets the wishes of the vehicle driver.
Another disadvantage is that the useful light component of the sensor signal can be evaluated, for the sake of triggering a windshield wiper, only at isolated times after the two intervals have elapsed. Dynamic evaluation or continuous evaluation of the useful light signal in real time during the longer first interval is accordingly impossible.
From German Patent Disclosure DE 42 17 390 A1, an apparatus for controlling a windshield wiper system is also known that has an optoelectronic rain sensor that decouples the ambient light component from the sensor signal by modulating the transmitter with a frequency in the range of greater than two kHz. The detected sensor signal, in the form of an alternating signal with a direct current component generated by the ambient light is separated by means of a known circuit, such as a phase-selective rectifier, into the useful light component and the interfering light component.
Other interfering factors of the ambient light, which arise for instance from the nonlinear characteristic curve of the radiation receiver of the rain sensor and cause a nonlinearity of the useful light signal under different ambient light conditions, are also precluded. To that end, the voltage dropping at a working resistor of the radiation receiver is detected and processed in a first operational amplifier to a correction variable, which is delivered to a second operational amplifier along with the useful light signal. The second operational amplifier eliminates the changes in the sensor signal caused by the ambient light by inputting the correction variable and outputs a useful light signal that has been linearized by the correction variable, and the useful light signal is delivered to a control stage for triggering a windshield wiper system of a motor vehicle.
A disadvantage here is that only the nonlinearity of the sensor signal caused by the radiation receiver is precluded. The nonlinear characteristic curve of the amplifier that is also present is not taken into account.
Another disadvantage is that the sensor signal is not linearized directly on being generated in the rain sensor but instead is linearized only after its further processing in a current to voltage converter, in the phase-selective rectifier, and in the amplifier, so that the sensor signal is amplified including the extraneous light component. This in turn contributes to worsening of the resolution of the useful light component of the sensor signal, as described above.